The 3rd Generation Partnership Project (3GPP) is responsible for the standardization of the Universal Mobile Telecommunication System (UMTS) and the fourth-generation wireless system commonly known as Long Term Evolution (LTE). The 3GPP work on LTE is also referred to as Evolved Universal Terrestrial Access Network (E-UTRAN). LTE is a technology for realizing high-speed packet-based communication that can reach high data rates both in the downlink (the link carrying transmissions from the base station to a mobile station) and in the uplink (the link carrying transmissions from a mobile station to the base station), and is thought of as a next-generation mobile communication system relative to UMTS. In order to support high data rates, LTE allows for a system bandwidth of 20 MHz, or up to 100 MHz when carrier aggregation is employed. LTE is also able to operate in several different frequency bands and can operate in at least Frequency-Division Duplex (FDD) and Time-Division Duplex (TDD) modes.
In LTE mobile broadband wireless communication systems, transmissions from base stations (referred to in 3GPP documentation as eNBs) to mobile stations (referred to as user equipment, or UEs) are sent using orthogonal frequency-division multiplexing (OFDM). OFDM splits the signal into multiple parallel sub-carriers in frequency. FIG. 1 illustrates the LTE downlink physical resource. The basic unit of transmission in LTE is a resource block (RB), which in its most common configuration consists of twelve subcarriers and seven OFDM symbols. The time interval of seven OFDM symbols is referred to as a “slot.” A unit of one subcarrier and one OFDM symbol is referred to as a resource element (RE), which can carry a modulated data symbol. Thus, an RB consists of 84 REs.
FIG. 2 illustrates the downlink subframe in LTE. An LTE radio subframe is composed of two slots in time and multiple resource blocks in frequency, with the number of RBs determining the bandwidth of the system. Furthermore, the two RBs in a subframe that are adjacent in time are denoted an RB pair. Currently, LTE supports standard bandwidth sizes of 6, 15, 25, 50, 75 and 100 RB pairs.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 milliseconds, each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 millisecond.
The signal transmitted by an eNB in a downlink subframe may be transmitted from multiple antennas, and the signal may be received at a UE that has multiple antennas. The radio channel distorts the signals transmitted from each of the multiple antenna ports. In order to demodulate any transmissions on the downlink, a UE thus relies on reference symbols (RS) that are transmitted on the downlink. These reference symbols and their positions in the time-frequency grid are known to the UE and can be used to determine channel estimates by measuring the effect of the radio channel on these symbols. As of Release 11 of the 3GPP specifications for LTE, there are multiple types of reference symbols. One important type is the common reference symbols (CRS), which are used for channel estimation during demodulation of control and data messages. The CRS are also used by the UE for synchronization, i.e., to align the UE's timing with the downlink signal as received from the eNB. The CRS occur once every subframe.
A key improvement to conventional cellular network deployments involves the deployment of relatively low-power “small cells” so as to overlay a conventional arrangement of co-called “macro cells.” The result is often referred to as a “heterogeneous network.” Heterogeneous networks, where the macro cells and the small cells have vastly different transmit powers, may be deployed in two main ways. In the first deployment type, the small cell layer and the macro cell layer share the same carrier frequencies. This approach creates interference between the two layers. In the second deployment type, the small cell layer and macro cell layer are on separate frequencies.
The network architecture for LTE allows messages to be sent between eNBs via an X2 interface. The eNB also can communicate with other nodes in the network, e.g., to the Mobility Management Entity (MME) via the S1 interface. In FIG. 3, the architecture involving E-UTRAN, i.e., the radio access network (RAN) and the core network (CN), is shown. In current specifications for LTE systems (see, e.g., “S1 Application Protocol,” 3GPP TS 36.413 v12.2.0, available at www.3gpp.org), methods are specified that allow some self-organizing network (SON) functionality, where an eNB can request information regarding another eNB via the MME.
As noted above, UEs use CRS transmitted by the eNB to synchronize to the eNB. Many features of 3GPP Long Term Evolution (LTE) technology, as well as of other technologies, benefit from the base stations (referred to as eNBs) in the system being synchronized with one another with respect to transmit timing and frequency. Synchronization of eNBs is typically done using a global navigation satellite system (GNSS), such as the global positioning system (GPS), or by using network-based methods such as IEEE 1588v2. However, when such methods are unavailable to an eNB, it is possible to use LTE reference signals transmitted by other eNBs to acquire synchronization. Such techniques are currently being discussed in 3GPP for small cells in LTE Rel-12, where a small cell can obtain synchronization from a macro cell or from other small cells.
Currently, a network interface-based signaling approach is used for synchronization purposes among eNBs. This is enabled by means of procedures known as the “S1: eNB Configuration Transfer” and “S1: MME Configuration Transfer” procedures, according to the following steps:                A first eNB, eNB1, generates an eNB Configuration Transfer message containing a SON Information Transfer information element (IE).        The MME receiving the eNB Configuration Transfer message forwards the SON Information Transfer IE towards a target eNB, eNB2, indicated in the IE, by means of the MME Configuration Transfer message.        If the SON Configuration Transfer IE contains a SON Information Request IE set to “Time synchronization Info,” the receiving eNB2 may reply with an eNB Configuration Transfer message towards the eNB1, including a SON Information Reply IE and Timing Synchronization Information IE, which contains Stratum Level and Synchronization Status of the sending node.        The MME receiving the eNB Configuration Transfer message from eNB2 forwards it to eNB1 by means of the MME Configuration Transfer message.        
In summary, within an eNB CONFIGURATION TRANSFER message from the eNB to the MME, it is possible to indicate a target eNB ID and the SON information that are required from that target eNB. The MME will therefore forward such an information request to the target eNB via a procedure called MME Configuration Transfer. Once the target eNB receives the request it will reply via the eNB Configuration Transfer towards the MME, which will include the information requested by the source eNB. The MME will forward the information requested to the source eNB by means of a new MME Information Transfer.
If a source eNB requests time synchronization information from a target eNB, the reply contained in the SON Configuration Transfer IE from target eNB to source eNB should include the above mentioned information elements (IEs):                Stratum level: This is the number of hops between the eNB and the synchronization source. That is, when the stratum level is M, the eNB is synchronized to an eNB whose stratum level is M−1, which in turn is synchronized to an eNB with stratum level M−2, and so on. The eNB with stratum level 0 is the synchronization source.        Synchronization status: This is a flag that indicates whether an eNB is currently in a synchronous or asynchronous state.OAM Architecture        
The management system architecture assumed for the present discussion is shown in FIG. 4. The node elements (NE), also referred to as eNodeB, are managed by a domain manager (DM), which is also referred to as the operation and support system (OSS). A DM may further be managed by a network manager (NM). Two NEs are interfaced by the X2 interface defined by the 3GPP specifications, whereas the interface between two DMs is referred to by the 3GPP specifications as the Itf-P2P interface. The management system may configure the network elements and may receive observations associated to features in the network elements. For example, a DM observes and configures NEs, while a NM observes and configures DMs, as well as NEs via the intermediate DMs.
By means of configuration via the DM, NM, and related interfaces, functions over the X2 and S1 interfaces can be carried out in a coordinated way throughout the RAN, eventually involving the Core Network, i.e., the MME and S-GWs.
Radio Interface-Based Synchronization (RIBS)
In recent progress in 3GPP RAN1's work, it was concluded that it would be beneficial, for synchronization purposes, to make use of patterns of time-frequency transmission resources that are selectively muted to ensure low interference, thus enabling RAN nodes in need of over-the-air synchronization to decode a synchronization reference signal that would otherwise be affected by neighbor cell interference and thus not usable. In particular, the resource elements in these muted patterns should be free from any reference signal or any other interfering signal's transmissions.
3GPP working group discussion documents R3-140997, “LS on Status of Radio-Interface Based Synchronization” (available at http://www.3gpp.org/FTP/tsg_ran/WG3_Iu/TSGR3_84/LSin/) and R1-142762, “LS on Radio Interface Based Synchronization” (available at http://www.3gpp.org/Liaisons/Outgoing_LSs/R1-meeting.htm), describe the agreements taken by RAN1 in terms of what characteristics such patterns should have.
In summary, the agreements state that the network should support the enabling of patterns of interference-protected time/frequency resources. These patterns can repeat themselves in time according to a period selected from a range specified in the latter of the two documents specified immediately above. It should be noted that these patterns are different from existing Almost Blank Subframes patterns, which are used for enhanced inter-cell interference coordination (eICIC). One difference is that in ABS patterns, reference signals are transmitted without interruption, which is one of the reasons why such patterns are made of so-called “Almost” blank subframes.
The 3GPP discussion documents identified above specify that the reference signals that a RAN node can use to achieve synchronization could be different, and that the interference protected patterns should therefore ensure protection towards all reference signals. In summary, the information from these documents that are relevant to the specification of signaling needed to make the radio interface-based synchronization mechanisms work are as follows.
Excerpts from R3-140997 (cited above):                Agreement:                    Specify listening RS(s) including RS pattern, and subframe periodicity, and offset, for both FDD and TDD                        Agreement:                    PRS and/or CRS is used as the listening RS for RIBS                            FFS: Down-select of listening RS                                    Subframe-level muting is supported for RIBS                        
Excerpts from R1-142762 (cited above):                For network listening, the following RS pattern is supported by configuration                    CRS only                            The number of CRS ports can be 1 or 2                                    CRS and PRS                            The number of CRS ports can be 1 or 2                                                The eNB should use one periodicity and offset of network listening RS that can be selected from the following recommended range of values                    A range of values (>=2) for the periodicity                            Choose all or a subset from [1280 ms, 2560 ms, 5120 ms, 10240 ms]                There is no consensus in RAN1 on the additional periodicities of 640 ms and 20480 ms                                    Values of offsets FFS                        The max number of hops is kept at 3.        
While the documents above provide a starting point for enabling interference protection for over-the-air synchronization measurements, further work is needed to provide complete solutions.